Comparison of Bond Strength of Metal and Ceramic Brackets Bonded with Conventional and High-Power LED Light Curing Units.

OBJECTIVES
The aim of this study was to evaluate the effect of conventional and high-power light emitting diode (LED) light curing units on shear bond strength (SBS) of metal and ceramic brackets to tooth surface.


MATERIALS AND METHODS
Forty sound bovine maxillary central incisors were used for the study. The teeth were divided into four groups (n=10). Teeth surfaces were etched with 37% phosphoric acid for 20 seconds. After applying a uniform layer of adhesive primer on the etched enamel, composite was placed on the base of brackets. The samples were light cured according to the manufacturer's instructions and thermocycled. The SBS was measured. The failure mode was scored using the adhesive remnant index (ARI).


RESULTS
The mean SBS of samples in groups A (high-power LED, metal bracket), B (high-power LED, ceramic bracket), C (conventional LED, metal bracket) and D (conventional LED, ceramic bracket) was 23.1±3.69, 10.7±2.06, 24.92±6.37 and 10.74±3.18MPa, respectively. The interaction effect of type of LED unit (high-power/conventional) and bracket type on SBS was not statistically significant (P=0.483). In general, type of LED unit did not affect SBS. Type of bracket significantly affected SBS (P<0.001). The ARI score was not significantly influenced by the interaction between the type of LED unit and bracket.


CONCLUSIONS
The obtained SBS is the same for both bracket types by use of high-power and conventional LED light curing units. Regardless of the type of LED unit, SBS of ceramic brackets was significantly lower than that of metal brackets.


INTRODUCTION
Appropriate bond strength between bracket and tooth surface is one of the most important aspects of orthodontic treatments [1,2]. Bonding of orthodontic brackets to enamel started in the mid 1960s [3,4]. Only auto-polymerizing materials were available at the time. Bonding of orthodontic brackets with visible light-cure adhesives was first reported by Tavas and Watts [5]. The light-cure adhesives were widely accepted due to their advantages in comparison with other chemical-cure adhesives. These advantages include high primary bond strength, better physical characteristics because of air inhibition phenomenon, user friendly application, extended working time for precise bracket placement and better removal of adhesive excess; but they have three major disadvantages namely being time-consuming, hindering light transmission and polymerization shrinkage [6,7]. Since then, several new methods using different composites and light-curing units have been introduced for this purpose. The halogen lamp, also known as quartz halogen and tungsten halogen lamp, has been used as lightcuring unit for many years [8,9], and is the most common source of visible blue light for dental applications. This lamp contains a blue filter to produce light of 400-500 nm wavelength [10]. The wide spectrum of action, easy use and low-cost maintenance are the most favorable characteristics of halogen light curing systems [9]. Despite their popularity, halogen light curing units have several disadvantages. For example, their light power output is 1% of the total electric energy consumed [11,12] [26,27]. Currently, some high-power LED curing units are able to emit light radiation with intensity of 1600-2000YmW/cm 2 , allowing shorter exposure times of six seconds for metal brackets [28]. In this study, the effect of conventional and high-power models of LED units on SBS of metal and ceramic brackets to tooth surfaces was evaluated.

MATERIALS AND METHODS
Forty sound bovine maxillary central incisors were used in this study. After extraction, the teeth were cleaned and immersed in 0.5% chloramine solution at 4°C for one week. They were divided into four groups of 10 teeth in each group. Next, teeth surfaces were etched with 37% phosphoric acid (Reliance; Itasca, IL, USA) for 20 seconds. After etching, the teeth were washed with water spray for approximately 10 seconds. The sample size (n=8 minimum samples for each group) was calculated with a power analysis in order to provide a statistical significance of alpha=0.05 and a standard deviation of 4.2 MPa using Minitab software. Sampling method in the study was consecutive. Bracket model and the type of light curing unit used for teeth were determined randomly. Group A: After checking correct conditioning of the enamel, metal brackets (American Orthodontics, Sheboygan, WI, USA) with a nominal base area of 11.3mm 2 were bonded with Transbond XT (3M ESPE, St. Paul, MN, USA), applying a uniform layer of adhesive primer on the etched enamel, and resin cement on the base of brackets. Brackets were placed in place and were pressed against the surface of the tooth. Excess cement was carefully removed with a dental probe, and the adhesive was high-power light-cured (2700mW/cm 2 ; Dentlight LLC, Plano, TX, USA) for four seconds (two seconds from mesial and two seconds from distal).  (Fig. 1).

Fig. 1: Testing shear bond strength of metal brackets
The results were obtained in kilogram-force, converted to Newtons and then to megapascals (MPa). After failure, the samples were observed under a stereomicroscope (SMZ 800; Nikon, Tokyo, Japan) at ×20 magnification to score the amount of remaining adhesive using the adhesive remnant index (ARI) [29]: 0=No adhesive remained on the tooth; 1=Less than 50% of adhesive remained on the tooth; 2=50% or more of the adhesive remained on the tooth surface; 3= 100% of the adhesive remained on the tooth, with a distinct impression of bracket mesh, corresponding to failure at the bracket-adhesive interface. Data were statistically analyzed using SPSS version 22.0.0 (SPSS Inc., Chicago, IL, USA). The mean, standard deviation, minimum and maximum values of SBS of metal and ceramic brackets to tooth surfaces using two models of light-curing units were computed and reported. The SBS data were analyzed using one-way ANOVA, followed by Tukey's post hoc test. Failure mode data were subjected to Kruskal-Wallis nonparametric test, followed by LSD post hoc test. Statistical significance was set at alpha=0.05.

RESULTS
According to the results presented in Table 1, the mean SBS of samples in groups A, B, C and D was 23.1±3.69, 10.7±2.06, 24.92±6.37 and 10.74±3.18MPa, respectively.  The mean and 95% confidence interval of SBS of metal and ceramic brackets to tooth surfaces using high-power and conventional LED light curing units are shown in Figure 2. Two-way ANOVA revealed a statistically significant difference in SBS among the groups (P=0.003). According to homogeneity of variances (Leven's test, P=0.131), Tukey's HSD test was applied for multiple comparisons.
The interaction effect of type of LED unit (highpower and conventional) and bracket on SBS was not statistically significant (P=0.483). In general, type of LED unit did not affect the SBS (P=0.467). According to the results of this study, type of bracket significantly affected the shear bond strength (P<0.001). Distribution of failure mode in the four experimental groups is presented in Table 2. The ARI score was not significantly influenced by the interaction effect of the type of LED (high-power and conventional) and bracket (P>0.05). The remaining adhesive in all ceramic brackets was less than 50%; while ARI scores in metal brackets were more diverse. There were no significant differences between ceramic and metal brackets in ARI scores (P>0.05).

DISCUSSION
In this study, we used SBS test, which is commonly used and has acceptable repeatability. This method has a high similarity to oral environment in terms of applied forces to samples. It has been discussed that different laboratory tests such as shear, compressive and tensile bond strength tests yield variable results and their findings cannot be compared [29,30] [11]. The advantages of LED units to halogen and plasma arc units include wireless system, lighter weight, smaller design and lifespan of 10,000 hours [22]. Plasma arc units are two to three times more expensive than halogen devices. Also, LED units are more expensive than halogen devices but cheaper than plasma arc. It has been reported that six seconds of curing by plasma arc creates bond strength as high as 40 seconds of radiation by halogen devices [40]. The high-power LED unit that was used in this study was cheaper than plasma arc but more expensive than conventional devices and due to remarkable reduction in chair time, the higher price is justified. Swanson et al. [41] showed that 40 seconds of curing by LED units results in a stronger bond, but 20 seconds of curing time also creates a bond strength higher than the required amount (>8MPa). In this study, 20 seconds of radiation was considered for the conventional unit for both bracket types and four seconds of curing for metal brackets and three seconds for ceramic brackets by high-power LED unit were considered. The mean bond strength for ceramic brackets was in the required range for both LED units; while the bond strength of metal brackets was higher than required. The lower bond strength of ceramic brackets could be due to the type of ceramic brackets used in this study. These types of brackets have no base design for micromechanical retention and also to reduce chemical bond strength; thus, chemical bond would only take place at the center of bracket base and this theorem was well observed when ARI scores were evaluated. Also, there was no difference in ARI scores of the two bracket types; however, the adhesive remnant pattern for all samples of ceramic brackets except one showed score 1 that means less than half of adhesive remained on tooth surface; but in use of metal brackets, all scores were found in samples. This could be because of base design of ceramic brackets, which was mentioned earlier. Adhesive bond failure occurred at the center of bracket base, while bond failure was cohesive at the peripheral parts.

CONCLUSION
The SBS of both brackets (metal and ceramic) by use of high-power and conventional LED units was the same. Therefore, using high-power LED units with shorter curing time is suggested. There was no difference in ARI scores with regard to bracket types or LED units used.